1. Field of The Invention
The present invention relates to a process for producing aromatic carbonates. More particularly, the present invention is concerned with a process for producing aromatic carbonates, which comprises transesterifying, in the presence of a metal-containing catalyst, a starting material selected from the group consisting of a dialkyl carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant selected from the group consisting of an aromatic monohydroxy compound, an alkyl aryl carbonate and a mixture thereof, characterized in that:
at least one type of catalyst-containing liquid is taken out, PA1 the catalyst-containing liquid being selected from the group consisting of: PA1 a portion of a high boiling point reaction mixture obtained by the above transesterification and containing the desired aromatic carbonate and the metal-containing catalyst, and PA1 a portion of a liquid catalyst fraction obtained by separating the desired aromatic carbonate from the high boiling point reaction mixture, PA1 each portion containing (A) high boiling point substance having a boiling point higher than the boiling point of the produced aromatic carbonate and containing (B) the metal-containing catalyst; PA1 (C) a functional substance capable of reacting with at least one component selected from the group consisting of the high boiling point substance (A) and the metal-containing catalyst (B) is added to the taken-out catalyst-containing liquid, to thereby obtain at least one reaction product selected from the group consisting of an (A)/(C) reaction product and a (B)/(C) reaction product; and PA1 the (B)/(C) reaction product is recycled to the reaction system, while withdrawing the (A)/(C) reaction product. PA1 in a process for producing aromatic carbonates which comprises transesterifying, in the presence of a metal-containing catalyst, a starting material selected from the group consisting of a dialkyl carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant selected from the group consisting of an aromatic monohydroxy compound, an alkyl aryl carbonate and a mixture thereof, PA1 when use is made of a process characterized in that: PA1 at least one type of catalyst-containing liquid is taken out, PA1 the catalyst-containing liquid being selected from the group consisting of a portion of a high boiling point reaction mixture obtained by the above transesterification and containing the desired aromatic carbonate and a metal-containing catalyst, and a portion of a liquid catalyst fraction obtained by separating the desired aromatic carbonate from the high boiling point reaction mixture, wherein each portion containing high boiling point substance (A) having a boiling point higher than the boiling point of the produced aromatic carbonate and containing the metal-containing catalyst (B); PA1 a functional substance (C) capable of reacting with at least one component selected from the group consisting of the high boiling point substance (A) and the metal-containing catalyst (B) is added to the taken-out catalyst-containing liquid, to thereby obtain at least one reaction product selected from the group consisting of an reaction product (A)/(C) and a reaction product (B)/(C); and PA1 the reaction product (B)/(C) is recycled to the reaction system directly or indirectly, while withdrawing the high boiling point substance without withdrawing the catalyst from the reaction system, PA1 disadvantageous phenomena, such as the accumulation of the high boiling point substance (A) in the reaction system which causes the discoloration of an ultimate aromatic polycarbonate (which is produced from an aromatic carbonate), can be prevented, so that a high purity aromatic carbonate can be stably produced for a prolonged period of time. The present invention has been completed, based on the above finding.
According to the process of the present invention, disadvantageous phenomena, such as the accumulation of the high boiling point substance (A) in the reaction system which causes the discoloration of an ultimate aromatic polycarbonate (which is produced from an aromatic carbonate), can be prevented without withdrawing the catalyst from the reaction system so that the desired aromatic carbonates having high purity can be produced stably for a prolonged period of time.
2. Prior Art
An aromatic carbonate is useful as a raw material for, e.g., the production of an aromatic polycarbonate (whose utility as engineering plastics has been increasing in recent years) without using poisonous phosgene. With respect to the method for the production of an aromatic carbonate, a method for producing an aromatic carbonate or an aromatic carbonate mixture is known, in which a dialkyl carbonate, an alkyl aryl carbonate or a mixture thereof is used as a starting material and an aromatic monohydroxy compound, an alkyl aryl carbonate or a mixture thereof is used as a reactant, and in which a transesterification reaction is performed between the starting material and the reactant.
However, since this type of transesterification is a reversible reaction in which, moreover, not only is the equilibrium biased toward the original system but the reaction rate is also low, the production of an aromatic carbonate by the above-mentioned method on an industrial scale is accompanied with great difficulties.
To improve the above-mentioned method, several proposals have been made, most of which relate to the development of a catalyst for increasing the reaction rate. As a catalyst for use in the method for producing an alkyl aryl carbonate, a diaryl carbonate or a mixture thereof by reacting a dialkyl carbonate with an aromatic hydroxy compound, there have been proposed various metal-containing catalysts, which include for example, a Lewis acid, such as a transition metal halide, or compounds capable of forming a Lewis acid, [see Unexamined Japanese Patent Application Laid-Open Specification No. 51-105032, Unexamined Japanese Patent Application Laid-Open Specification No. 56-123948 and Unexamined Japanese Patent Application Laid-Open Specification No. 56-123949 (corresponding to West German Patent Application Publication No. 2528412, British Patent No. 1499530 and U.S. Pat. No. 4,182,726)], a tin compound, such as an organotin alkoxide or an organotin oxide [Unexamined Japanese Patent Application Laid-Open Specification No. 54-48733 (corresponding to West German Patent Application Publication No. 2736062), Unexamined Japanese Patent Application Laid-Open Specification No. 54-63023, Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110 and West German Patent Application Publication No. 3445552), Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704 and West German Patent Application Publication No. 3445555), Unexamined Japanese Patent Application Laid-Open Specification No. 62-277345, and Unexamined Japanese Patent Application Laid-Open Specification No. 1-265063 (corresponding to European Patent Publication No. 338760 and U.S. Pat. No. 5,034,557)], salts and alkoxides of an alkali metal or an alkaline earth metal (Unexamined Japanese Patent Application Laid-Open Specification No. 56-25138), lead compounds (Unexamined Japanese Patent Application Laid- Open Specification No. 57-176932), complexes of a metal, such as copper, iron or zirconium (Unexamined Japanese Patent Application Laid-Open Specification No. 57-183745), titanic acid esters [Unexamined Japanese Patent Application Laid-Open Specification No. 58-185536 (corresponding to U.S. Pat. No. 4,410,464 and West German Patent Application Publication No. 3308921)], a mixture of a Lewis acid and protonic acid [Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501 and West German Patent Application Publication No. 3445553)], a compound of Sc, Mo, Mn, Bi, Te or the like [Unexamined Japanese Patent Application Laid-Open Specification No. 1-265064 (corresponding to European Patent Publication No. 0 338 760 A1 and U.S. Pat. No. 5,034,557)], and ferric acetate (Unexamined Japanese Patent Application Laid-Open Specification No. 61-172852).
As a catalyst for use in the method for producing a diaryl carbonate by a same-species intermolecular transesterification, wherein an alkyl aryl carbonate is disproportionated to a dialkyl carbonate and a diaryl carbonate, there have been proposed various catalysts, which include for example, a Lewis acid and a transition metal compound which is capable of forming a Lewis acid [see Unexamined Japanese Patent Application Laid-Open Specification No. 51-75044 (corresponding to West German Patent Application Publication No. 2552907 and U.S. Pat. No. 4,045,464)], a polymeric tin compound [Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110 and West German Patent Application Publication No. 3445552)], a compound represented by the formula R--X(=O)OH (wherein X is selected from Sn and Ti, and R is selected from monovalent hydrocarbon residues) [Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704 and West German Patent Application Publication No. 3445555)], a mixture of a Lewis acid and protonic acid [Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501 and West German Patent Application Publication No. 3445553)], a lead catalyst [Unexamined Japanese Patent Application Laid-Open Specification No. 1-93560 (corresponding to U.S. Pat. No. 5,166,393)], a titanium or zirconium compound (Unexamined Japanese Patent Application Laid-Open Specification No. 1-265062), a tin compound [Unexamined Japanese Patent Application Laid-Open Specification No. 1-265063 (corresponding to U.S. Pat. No. 5,034,557 and European Patent Publication No. 0 338 760)], and a compound of Sc, Mo, Mn, Bi, Te or the like [Unexamined Japanese Patent Application Laid-Open Specification No. 1-265064 (corresponding to U.S. Pat. No. 5,034,557 and European Patent Publication No. 0 338 760)].
Another attempt for improving the yield of aromatic carbonates in these reactions consists in biasing the equilibrium toward the product system as much as possible, by modifying the mode of the reaction process. For example, there have been proposed a method in which by-produced methanol is distilled off together with an azeotrope forming agent by azeotropic distillation in the reaction of a dimethyl carbonate with phenol [see Unexamined Japanese Patent Application Laid-Open Specification No. 54-48732 (corresponding to West German Patent Application Publication No. 2736063 and U.S. Pat. No. 4,252,737) and Unexamined Japanese Patent Application Laid-Open Specification No. 61-291545], and a method in which by-produced methanol is removed by adsorbing the same onto a molecular sieve [Unexamined Japanese Patent Application Laid-Open Specification No. 58-185536 (corresponding to U.S. Pat. No. 4,410,464 and West German Patent Application Publication No. 3308921)].
Further, a method is known in which an apparatus comprising a reactor having provided on the top thereof a distillation column is employed in order to separate and distill off alcohols (by-produced in the course of the reaction) from a reaction mixture obtained in the reactor. [With respect to this method, reference can be made to, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 56-123948 (corresponding to U.S. Pat. No. 4,182,726 and West German Patent Application Publication No. 2528412), Unexamined Japanese Patent Application Laid-Open Specification No. 56-25138, Unexamined Japanese Patent Application Laid-Open Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110 and West German Patent Application Publication No. 3445552), Unexamined Japanese Patent Application Laid-Open Specification No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704 and West German Patent Application Publication No. 3445555), Unexamined Japanese Patent Application Laid-Open Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501 and West German Patent Application Publication No. 3445553), Unexamined Japanese Patent Application Laid-Open Specification No. 61-172852, Unexamined Japanese Patent Application Laid-Open Specification No. 61-291545, and Unexamined Japanese Patent Application Laid-Open Specification No. 62-277345.]
As more preferred methods for producing an aromatic carbonate, the present inventors previously developed a method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a by-produced alcohol from an upper portion of the distillation column by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced alkyl aryl carbonate from a lower portion of the distillation column [see Unexamined Japanese Patent Application Laid-Open Specification No. 3-291257 (corresponding to U.S. Pat. No. 5,210,268 and European Patent Publication No. 0 461 274)], and a method in which an alkyl aryl carbonate is continuously fed to a continuous multi-stage distillation column to effect a continuous transesterification reaction in the distillation column, while continuously withdrawing a low boiling point reaction mixture containing a by-produced dialkyl carbonate by distillation and continuously withdrawing a high boiling point reaction mixture containing a produced diaryl carbonate from a lower portion of the distillation column [see Unexamined Japanese Patent Application Laid-Open Specification No. 4-9358 (corresponding to U.S. Pat. No. 5,210,268 and European Patent Publication No. 0 461 274)]. These methods for the first time realized efficient, continuous production of an aromatic carbonate. Thereafter, various methods for continuously producing an aromatic carbonate have further been developed, based on the above-mentioned methods developed by the present inventors. Examples of these methods include a method in which a catalytic transesterification reaction is performed in a column reactor [see Unexamined Japanese Patent Application Laid-Open Specification No. 6-41022 (corresponding to U.S. Pat. No. 5,362,901 and European Patent Publication No. 0 572 870), Unexamined Japanese Patent Application Laid-Open Specification No. 6-157424 (corresponding to U.S. Pat. No. 5,334,724 and European Patent Publication No. 0 582 931), Unexamined Japanese Patent Application Laid-Open Specification No. 6-184058 (corresponding to U.S. Pat. No. 5,344,954 and European Patent Publication No. 0 582 930)], a method in which use is made of a plurality of reactors which are connected in series [Unexamined Japanese Patent Application Laid-Open Specification No. 6-234707 (corresponding to U.S. Pat. No. 5,463,102 and European Patent Publication No. 0 608 710 A1), and Unexamined Japanese Patent Application Laid-Open Specification No. 6-263694], a method in which a bubble tower reactor is used [Unexamined Japanese Patent Application Laid-Open Specification No. 6-298700 (corresponding to U.S. Pat. No. 5,523,451 and European Patent Publication No. 0 614 877)], and a method in which a vertically long reactor vessel is used (Unexamined Japanese Patent Application Laid-Open Specification No. 6-345697).
Also, there have been proposed methods for producing an aromatic carbonate stably for a prolonged period of time on a commercial scale. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 6-157410 (corresponding to U.S. Pat. No. 5,380,908 and European Patent Publication No. 0 591 923 A1) discloses a method for producing aromatic carbonates from a dialkyl carbonate and an aromatic hydroxy compound, which comprises continuously supplying a mixture of raw materials and a catalyst to a reactor provided with a distillation column thereon to effect a transesterification reaction in the reactor, while continuously withdrawing a by-produced aliphatic alcohol from the reactor through the distillation column by distillation so as to keep the aliphatic alcohol concentration of the reaction system at 2% by weight or less. This prior art document describes that, by this method, continuous production of an aromatic carbonate can be performed in a stable manner. The object of this method is to avoid the deposition of the catalyst in the distillation column. Further, Patent Application prior-to-examination Publication (Kohyo) No. 9-11049 (corresponding to WO 97/11049) discloses a process for producing an aromatic carbonate, in which the transesterification is conducted while maintaining a weight ratio of an aromatic polyhydroxy compound and/or a residue thereof to the metal component of the metal-containing catalyst at 2.0 or less, with respect to a catalyst-containing liquid-phase mixture in a system for the transesterification, so that the desired aromatic carbonates can be produced stably for a prolonged period of time without suffering disadvantageous phenomena, such as the deposition of the catalyst.
On the other hand, it is known that when an aromatic carbonate is produced by transesterification, high boiling point substances are likely to be by-produced. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 61-172852 discloses that when diphenyl carbonate is produced by a transesterification of dimethyl carbonate with phenol, an impurity having a boiling point equal to or higher than the boiling point of the produced diphenyl carbonate is by-produced, and that the impurity is caused to enter the diphenyl carbonate and causes the discoloration of an ultimate product, such as an aromatic polycarbonate. This prior art document does not dis-close an example of the impurity having a boiling point equal to or higher than the boiling point of the produced diphenyl carbonate; however, as an example of the impurity, there can be mentioned an aryloxycarbonyl-(hydroxy)-arene which is produced as an isomer of a diaryl carbonate by Fries rearrangement. More specifically, when diphenyl carbonate is produced as the diaryl carbonate, a phenyl salicylate can be mentioned as an example of the aryloxycarbonyl-(hydroxy)-arene. Phenyl salicylate is a high boiling point substance whose boiling point is 4 to 5.degree. C. higher than the boiling point of the diphenyl carbonate.
In this case, when the transesterification is conducted for a long period of time, the above-mentioned high boiling point substance accumulates in the reaction system and the amount of the impurity mixed into the ultimate aromatic carbonate tends to increase, so that the purity of the ultimate aromatic carbonate is lowered. Further, as the amount of the high boiling point substance in the reaction mixture increases, the boiling point of the reaction mixture rises, so that the by-production of the high boiling point substance is accelerated, thus rendering it difficult to produce desired aromatic carbonates stably for a prolonged period of time. As a measure for solving the problems, it is conceivable to withdraw a high boiling point substance-containing reaction mixture from the reaction system, thereby preventing the accumulation of the high boiling point substance in the reaction system. However, by this measure, a disadvantage is brought about in that, when a catalyst which is soluble in the reaction liquid is used, both the catalyst and the high boiling point substance are present in a state dissolved in the reaction mixture, so that, for separating the catalyst from the high boiling point substance by a conventional distillation method, it is necessary to heat the reaction mixture at high temperatures, leading to a further increased formation of by-products. Therefore, it is difficult to separate the catalyst from the high boiling point substance. This means that the withdrawal of the high boiling point substance from the reaction system is inevitably accompanied by the discharge of the catalyst. Accordingly, for continuing the reaction, it is necessary to supply a fresh catalyst to the reaction system. As a result, a large quantity of the catalyst is needed.